Add a display density parameter.

This change add a display density parameter to
vm_tools.cicerone.LaunchContainerApplicationRequest.

This is used to enlarge Crostini windows for some older Linux apps that
don't show well on HiDPI displays by user preferences.

BUG=chromium:839242
TEST=New proto buffer field added and it still compiles.

Change-Id: I963f1bf60281b16110e515891e3a44a3fab6505e
Reviewed-on: https://chromium-review.googlesource.com/1226168


Commit-Ready: Tim Zheng <timzheng@chromium.org>
Tested-by: Tim Zheng <timzheng@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Reviewed-by: David Reveman <reveman@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>